Secondary bobbins of this kind are known, for example from European Patent Application EP-A 0 375 502.
In internal combustion engines with controlled ignition, the combustion of the gaseous mixture in the cylinder is induced by the spark that is produced between the electrodes of a spark plug.
In order to produce this spark, the terminals of the spark plug are connected to the ends of the secondary (high-voltage) winding of a transformer, such as an ignition coil, whose primary winding is connected to a voltage source by way of a switch, such as a transistor.
With this switch closed, an electric current circulates in the primary winding. If at a given moment the switch is then opened, an abrupt overvoltage occurs in the primary winding, which by induction generates a voltage spike within the secondary winding. Once this voltage reaches a sufficient value, the spark is produced, causing the combustible mixture to ignite.
The voltage at the terminals of the secondary winding may be as high as several tens of thousands of volts. The aforementioned fins, by defining separate winding compartments, make it possible to limit the risk of a spark developing between two turns of this winding. In fact, the voltage between the starting turn and the ending turn of the same compartment is limited to several thousand volts, in the typical case where the number of compartments is on the order of about 10.
Nevertheless, the beginning and end of a compartment is formed by a slot in which each of the fins define this compartment, and so the voltage between two turns located facing one another on either side of this slot in two adjacent compartments still remains on the order of several thousand volts, so that the risks that a spark may develop are nevertheless not completely averted.
The aforementioned document proposes to overcome this disadvantage by providing insulating compartments between the winding compartments in such a way as to increase the distance between the turns of two successive winding compartments.
Such an arrangement nevertheless has the disadvantage of increasing the axial size of the secondary bobbin, and consequently of the ignition coil that includes it.